1. Field of the Invention
The present invention relates to a preparation method of reduced and N-doped graphene oxide and the reduced and N-doped graphene oxide prepared thereby, more precisely the method for preparing the reduced and N-doped graphene oxide using low temperature process.
2. Description of the Related Art
Graphene is as thick as one carbon atom and has a 2-dimensional tabular structure. Graphene is composed of sp2 carbon hybrid structure, so that it has excellent electrical, thermal, and mechanical properties. Also, graphene has a large surface area of 300˜400 m2/g, so that it can favorably applied to various fields such as transistor, transparent electrode, sensor, polymer, supercapacitor, secondary cell, and energy storage materials such as hydrogen storage material, etc.
Graphene is generally prepared by chemical exfoliation chemical exfoliation. This method has the advantage of usability for the mass-production of graphene and of low production costs since the method uses graphite as a raw material. The chemical exfoliation can provide graphene in the form of colloid so regular in liquid phase as to be applied in various forms. In addition, graphene can obtain other properties by the chemical modification.
However, the sp2 carbon hybrid structure of graphene prepared by such chemical exfoliation, which is graphene oxide, is easily destructed in the course of oxidation and sonication, resulting in the compromised physical properties. Thus, in that case, it is necessary to recover the sp2 carbon hybrid structure through chemical reduction.
When the graphene oxide prepared by chemical exfoliation is reduced chemically, the functional groups including oxygen linked to graphene oxide are eliminated and thereafter the sp2 carbon hybrid structure is recovered, resulting in the reduced graphene oxide having the original graphene-like structure.
Studies have been undergoing not only with graphene but also with graphene oxide (GO) and the reduced graphene oxide (RGO) as alternatives for graphene.
The reducing agent to reduce graphene oxide is represented by hydrazine (NH2NH2), which was once described in Korean Patent No. 10-1084975. Particularly, in this patent document, the method for reducing graphene oxide by heating, the method for reducing graphene oxide by supplying hydrogen additionally, and the method for reducing graphene oxide by treating the reducing agent prepared by mixing hydrazine and ammonia are described. However, the said patent depended on the general graphene oxide reduction method generally used so far and therefore the produced graphene film contained many functional groups including oxygen, which limits the method in reducing graphene oxide efficiently. Besides, the above method required a high temperature reaction.
Graphene composed of carbon only has high conductivity because of the n-n conjugation structure. However, when some of carbons in the graphene are substituted with nitrogen that has one more electrons than carbon, the conductivity of graphene increases.
Therefore, various methods have been proposed to synthesize N-doped graphene. Among them, chemical vapor deposition and N-plasma based method are the most representative ones.
However, these methods have a difficulty in mass-production, which have been studied in many aspects so far.
Korean Patent No. 10-1400441 presents an example of the method for preparing N-doped graphene and the N-doped graphene prepared thereby. Particularly, the patent provides the method for preparing N-doped graphene composed of the steps of mixing hexamethylene tetramine in graphene oxide dispersion; and inducing hydrothermal reaction of the graphene oxide mixed with the said amine derivative. That is, N-doped graphene has been prepared by hydrothermal reaction performed with the amine compound at the temperature of at least 100° C. So, it is obvious that this method requires a high temperature reaction for N-doping.
The present inventors studied to establish a method for preparing reduced and at the same time N-doped graphene oxide at a low temperature. In the course of the study, the inventors developed a preparation method of the reduced and N-doped graphene oxide comprising the steps of preparing the mixed solution containing graphene oxide (GO) and tetramethylammonium hydroxide (TMAH), and heating the mixed solution. The present inventors completed this invention by confirming that the reduced and N-doped graphene oxide could be satisfactorily prepared even at a low temperature by the said method.